1. Field of the Invention
The present invention relates to a crystal for an X-ray or .gamma.-ray scintillator. More particularly, the invention relates to a crystal for a scintillator having a short decay time and a high sensitivity.
2. Brief Description of the Prior Art
A single crystal of bismuth germanate (Bi.sub.4 Ge.sub.3 O.sub.12, hereinafter referred to as "BGO") heretofore used as a crystal for an X-ray or .gamma.-ray scintillator are characterized in that (i) since the atomic numbers of the constituent elements are large and the density of the crystal is high, the absorption coefficient to radiations is high, (ii) the decay time is very short (the time necessary for attenuation to 0.1% is 30 milliseconds) and (iii) the crystal is nonhygroscopic. Because of these merits, this single crystal is used as a scintillator crystal for X-ray CT (computed Tomography) and a medical instrument such as a positron camera, though the sensitivity is relatively low. NaI or thallium (Tl)-incorporated NaI, which has been used for the above-mentioned object, has a sensitivity about 10 times as high as that of the crystal of BGO, but the decay time is very long (the time necessary for attenuation to 0.1% is 350 milliseconds). Accordingly, NaI or Tl-incorporated NaI has a defect that the image becomes obscure. Accordingly, it has been desired to increase the sensitivity of BGO while not degrading the merit of a short decay time. The scintillation conversion efficiency (light output) of the conventional cyrstal of BGO is only 10% of that of NaI or the like, and in order to reduce doses of radiations applied to human bodies, development of a crystal of BGO having a high scintillation conversion efficiency has been desired.
The scintillation center of the crystal of BGO is the ion of Bi as the constituent element. According to the conventional technique, the zone-refined starting material is used for increasing the purity and removing impurity elements having bad influences and the scintillation conversion efficiency is thus improved. However, a long time is necessary for the zone refining of the starting material for attaining a desired high purity and the loss of the starting material is large. For example, the final yield is as low as about 35%.
The following references are cited to show the state of the art:
(1) M. J. Weber and R. R. Moncham; Journal of Applied Physics, vol. 44 (1973), pages 5495-5499;
(2) O. H. Nestor and C. Y. Huang; IEEE Transactions on Nuclear Science, vol. NS-22, February 1975, pages 68-71;
(3) Z. H. Cho and M. R. Farukhi; Journal of Nuclear Medicine;vol. 18 (1977), pages 840-844.